The present invention relates to an orthodontic arch wire.
A variety of malocclusions are corrected by the use of orthodontic arch wires. In corrections using arch wires, brackets are secured to the patient""s teeth and, in each of a number of stages during the course of treatment, an arch wire is secured in the brackets. As treatment progresses, the arch wire which is used more closely approaches the ideal arch form. The arch wires progressively correct misalignments of the patient""s teeth.
It is known in the art to use an arch wire of non-circular cross-section, e.g., rectangular or trapezoidal, in brackets with rectangular slots so that the arch wire cannot rotate in the slot and that torque can be applied to the teeth by the arch wire through the brackets. One problem with the use of non-circular cross-sectional arch wires is that the arch wire tends to bind in the brackets on the posterior teeth as a result of excessive friction. It is known from my prior U.S. Pat. No. 4,424,033, the contents of which are incorporated by reference herein in their entireties, to use an arch wire having posterior segments of circular cross-section and an anterior segment, at least a part of which is rectangular or trapezoidal in cross-section. The arch wire described in my prior U.S. Pat. No. 4,424,033 has a unitary construction, i.e., is formed from a single or unitary length of wire. Manufacture of such a wire has proved difficult in that it is generally necessary to start with an arch wire of non-circular, e.g., rectangular or trapezoidal, cross-section, and work the posterior segments of the wire to provide a circular cross-section.
It is known from U.S. Pat. No. 4,412,819 to Cannon to provide an orthodontic arch wire formed by uniting a central segment of relatively resilient wire with end or posterior segments of a different and more rigid wire to provide an arch wire having an anterior segment made of a material having a flexural rigidity which is lower than the flexural rigidity of the material forming the posterior segments. The contents of U.S. Pat. No. 4,412,819 to Cannon are incorporated herein by reference in their entireties. However, an arch wire having a more resilient anterior segment is advantageous only in early arch treatment placements when irregular anterior teeth would benefit from being more easily attached to the arch. Moreover, using a resilient anterior segment tends to cause constriction of posterior dental elements. For example, if a resilient Nixe2x80x94Ti anterior segment is used with elastic pull bilaterally, the anterior radius tends to become V-shaped, constricting the posterior segments. Moreover, the use of dissimilar metals for the anterior and posterior segments makes it difficult to solder or braze the segments together or to a joining element since different fluxes and solders are required for the different metals. Finally, it is often desirable to provide a metal (e.g., gold) plating on the arch wire, this is difficult to do when dissimilar metals are used since the conditioning acids or other baths needed to activate the metal surfaces are different for different metals.
The present invention concerns an orthodontic arch wire that has a generally parabolic shape. The arch wire includes a curved anterior segment and a pair of posterior segments connected to and extending from respective ends of the curved anterior segment. The curved anterior segment and the pair of posterior segments comprise three discrete pieces having substantially the same flexural rigidity. The three pieces are joined together to form the generally parabolic shape.